Sulfonyl-Stabilized Allylic Norbornenyl and Norbornyl Carbanions: Structure and Stereoselectivity of Reaction with Electrophiles

Abstract

The structures of the norbornenyl and norbornyl sulfones exo‐5, endo‐5 and endo‐6 have been determined experimentally, by X‐ray analysis, and theoretically by ab initio calculations (HF/6–31+G*). X‐ray crystal structure analyses of the lithiated allylic norbornenyl and norbornyl sulfones endo‐3/ent‐endo‐3·2diglyme and endo‐4/ent‐endo‐4·2diglyme revealed dimeric O–Li contact ion pairs devoid of C–Li bonds. The anions of endo‐3/ent‐endo‐3·2diglyme and endo‐4/ent‐endo‐4·2diglyme adopt both the endo conformation (C2–S) and are characterized by in the exo direction pyramidalized anionic C atoms. The degree of pyramidalization of the C2 atom of 3 is higher than that of 4. Ab initio optimizations (HF/6–31+G*) of the structures of the anions of methylenenorbornene I and methylenenorbornane II resulted in local minima featuring non‐planar C2 atoms which are pyramidalized in the exo direction in both cases, but to different degrees. In both cases cryoscopy of 3 and 4 in THF at –108.5 °C revealed approximately 1:1 mixtures of monomers and dimers. The sulfones exo‐5, endo‐5, exo‐6 and endo‐6 as well as the lithiosulfones 3 and 4 were studied by NMR spectroscopy. 1H‐NMR (400 MHz), 13C‐NMR (100 MHz) and 6Li‐NMR (44 MHz) spectroscopy of 3 and 4 at –100 °C in [D]8THF revealed in each case only one set of signals, independent of the configuration of the starting sulfones. This indicates in both cases that attainment of both the monomer‐dimer and the endo/exo equilibria of 3 and 4 is fast on the NMR time scale. According to 6Li{1H}‐ and 1H{1H}‐NOE experiments of 3 and 4 the monomeric and dimeric species endo‐3 and endo‐4, having endo anions, seem to be preferred in THF solution. Ab initio calculations of the anions of 3 and 4 resulted in structures endo‐3(–Li+), exo‐3(–Li+), endo‐4(–Li+) and exo‐4(–Li+) (HF/6–31+G*), whose atomic point charges were calculated by the method of Kollman et al. The C2 atoms of endo‐3(–Li+) and endo‐4(–Li+) are pyramidalized in the exo direction whereas the C2 atoms of exo‐3(–Li+) and exo‐4(–Li+) are pyramidalized in the endo direction. According to the calculations, the endo anions are more stable than the exo anions. There is good agreement between the optimized structures of the free anions and the experimentally determined structures of the anions of the contact ion pairs in the crystal. Reactions of 3 and 4 with DX, MeI, EtI, nPrI and nHeI occurred at the C2 atom under the selective formation of the corresponding endosulfones endo‐8a–e and endo‐9a–e, respectively, in all cases. Thus, an earlier report on the selective formation of the exosulfone exo‐9b in the reaction of 4 with MeI has to be revised. Product ratios were independent of the configuration of the starting sulfones and varied with the nature of the electrophile. Selectivities were highest in the case of the norbornyl species 4. Reaction of 3 with PhCHO occurred at the α position (C2) to afford the alcohols endo‐8f and exo‐8f (88:12) as single diastereomers and at higher temperatures at the γ position (C8), whereas reaction of ...

Lithium Salts of Conformationally Constrained and Restricted Chiral Allylic α-Sulfonyl Carbanions − A Joint Study of their Structures, Dynamics, and Stereoselectivities

“…Scheme Base-catalyzed equilibration of sulfones exo-9 and endo-9 with NaOCD 3 in CD 3 OD in both cases gave a mixture of the exo and endo isomers, both fully deuterated at their α-positions (Ͼ98% D), in a ratio of 67:33. Thus, the exo isomer is, as in the case of the corresponding S-phenyl derivative, [7] more stable than the endo isomer.…”

“…Figure 1). [7] A further important assumption made was that the sulfonyl groups of endo and exo 1 and 2 directed electrophiles to attack the Cα atoms with high selectivities from the side syn to the O atoms and thereby suppressed crossover reactions (k 4 , k 5 ഠ 0). Unfortunately, however, kinetic quenching (k 2 , k 3 ϾϾ k 1 , k Ϫ1 ) [13b] of the endo and exo diastereomers of 1 and 2, which would have given important clues as to the validity of these assumptions, was not possible because of their fast equilibration.…”

“…Further goals of the investigation of 3 and 4 were the determination of the structures of their monomers and dimers in solution and of their monomers in the crystal. In order to study the influence of the methylene group in the 7-position upon the structure and dynamics of 3, carbanion salt 5, which bears a benzhydrylidene group at C (7), and carbanion salt 6, with an O atom in the 7-position, [14] were included in this investigation.…”

A Study on the Reaction of Lithiated Allyl Systems with CO2, Isocyanates and Isothiocyanates

“…In this article, aggregational effects can be neglected since we recently succeeded in showing that the sulfur-stabilized allyllithium compounds considered here are monomeric in THF at Ϫ108°C and, in addition, exist exclusively in an endo-conformation. [2,20,21] Furthermore, the sulfur heteroanalogues are substituted by the same group (phenyl) in order to permit direct comparison of the effect of the sulfur oxidation state upon the regioselectivity. Since the substituents on the phosphorus atom may have a steric influence on the course of the reaction, the ethyl derivative of allyl phosphonate was chosen throughout.…”

“…The colorless liquid (E)-4-(diethoxyphosphoryl)but-2-enoic acid (20) was purified by column chromatography with ethyl acetate/hexane (1:10 followed by 1:5). White crystals suitable for X-ray crystallography were formed.…”